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Subject[PATCH 03/11] exec: Compute file based creds only once

Move the computation of creds from prepare_binfmt into begin_new_exec
so that the creds can be computed only onc.

I have looked through the kernel and verified none of the binfmts
look at bprm->cred directly so computing the bprm->cred later
should be safe.

Rename preserve_creds to execfd_creds to make it clear that the creds
should be derived from the executable file descriptor.

Remove active_secureexec and active_per_clear and use secureexec and
per_clear respectively. The active versions of these variables were
only necessary to allow their values to be recomputed from scratch
for each value of bprm->file.

Remove the now unnecessary work from bprm_fill_uid to reset the
bprm->cred->euid and bprm->cred->egid, and add a small comment
about what bprm_fill_uid now does.

Remove the now unnecessary work in cap_bprm_creds_from_file to
reset the ambient capabilities, and add a small comment
about what cap_bprm_creds_from_file does.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
fs/binfmt_misc.c | 2 +-
fs/exec.c | 65 +++++++++++++++++------------------
include/linux/binfmts.h | 12 ++-----
include/linux/lsm_hook_defs.h | 2 +-
include/linux/lsm_hooks.h | 19 +++++-----
include/linux/security.h | 8 ++---
security/commoncap.c | 12 +++----
security/security.c | 4 +--
8 files changed, 57 insertions(+), 67 deletions(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index 53968ea07b57..bc5506619b7e 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -192,7 +192,7 @@ static int load_misc_binary(struct linux_binprm *bprm)

bprm->interpreter = interp_file;
if (fmt->flags & MISC_FMT_CREDENTIALS)
- bprm->preserve_creds = 1;
+ bprm->execfd_creds = 1;

retval = 0;
ret:
diff --git a/fs/exec.c b/fs/exec.c
index 221d12dcaa3e..091ff6269610 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -72,6 +72,8 @@

#include <trace/events/sched.h>

+static int bprm_creds_from_file(struct linux_binprm *bprm);
+
int suid_dumpable = 0;

static LIST_HEAD(formats);
@@ -1304,6 +1306,11 @@ int begin_new_exec(struct linux_binprm * bprm)
struct task_struct *me = current;
int retval;

+ /* Once we are committed compute the creds */
+ retval = bprm_creds_from_file(bprm);
+ if (retval)
+ return retval;
+
/*
* Ensure all future errors are fatal.
*/
@@ -1354,7 +1361,7 @@ int begin_new_exec(struct linux_binprm * bprm)
me->flags &= ~(PF_RANDOMIZE | PF_FORKNOEXEC | PF_KTHREAD |
PF_NOFREEZE | PF_NO_SETAFFINITY);
flush_thread();
- if (bprm->per_clear || bprm->active_per_clear)
+ if (bprm->per_clear)
me->personality &= ~PER_CLEAR_ON_SETID;

/*
@@ -1365,13 +1372,6 @@ int begin_new_exec(struct linux_binprm * bprm)
*/
do_close_on_exec(me->files);

- /*
- * Once here, prepare_binrpm() will not be called any more, so
- * the final state of setuid/setgid/fscaps can be merged into the
- * secureexec flag.
- */
- bprm->secureexec |= bprm->active_secureexec;
-
if (bprm->secureexec) {
/* Make sure parent cannot signal privileged process. */
me->pdeath_signal = 0;
@@ -1589,20 +1589,12 @@ static void check_unsafe_exec(struct linux_binprm *bprm)

static void bprm_fill_uid(struct linux_binprm *bprm)
{
+ /* Handle suid and sgid on files */
struct inode *inode;
unsigned int mode;
kuid_t uid;
kgid_t gid;

- /*
- * Since this can be called multiple times (via prepare_binprm),
- * we must clear any previous work done when setting set[ug]id
- * bits from any earlier bprm->file uses (for example when run
- * first for a setuid script then again for its interpreter).
- */
- bprm->cred->euid = current_euid();
- bprm->cred->egid = current_egid();
-
if (!mnt_may_suid(bprm->file->f_path.mnt))
return;

@@ -1629,19 +1621,38 @@ static void bprm_fill_uid(struct linux_binprm *bprm)
return;

if (mode & S_ISUID) {
- bprm->active_per_clear = 1;
+ bprm->per_clear = 1;
bprm->cred->euid = uid;
}

if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
- bprm->active_per_clear = 1;
+ bprm->per_clear = 1;
bprm->cred->egid = gid;
}
}

+/*
+ * Compute brpm->cred based upon the final binary.
+ */
+static int bprm_creds_from_file(struct linux_binprm *bprm)
+{
+ struct file *file = bprm->file;
+ int retval;
+
+ /* Compute creds from the executable passed to userspace? */
+ if (bprm->execfd_creds)
+ bprm->file = bprm->executable;
+
+ bprm_fill_uid(bprm);
+ retval = security_bprm_creds_from_file(bprm);
+ bprm->file = file;
+
+ return retval;
+}
+
/*
* Fill the binprm structure from the inode.
- * Check permissions, then read the first BINPRM_BUF_SIZE bytes
+ * Read the first BINPRM_BUF_SIZE bytes
*
* This may be called multiple times for binary chains (scripts for example).
*/
@@ -1649,20 +1660,6 @@ static int prepare_binprm(struct linux_binprm *bprm)
{
loff_t pos = 0;

- /* Can the interpreter get to the executable without races? */
- if (!bprm->preserve_creds) {
- int retval;
-
- /* Recompute parts of bprm->cred based on bprm->file */
- bprm->active_secureexec = 0;
- bprm->active_per_clear = 0;
- bprm_fill_uid(bprm);
- retval = security_bprm_repopulate_creds(bprm);
- if (retval)
- return retval;
- }
- bprm->preserve_creds = 0;
-
memset(bprm->buf, 0, BINPRM_BUF_SIZE);
return kernel_read(bprm->file, bprm->buf, BINPRM_BUF_SIZE, &pos);
}
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 89231a689957..39f6b5a7ace7 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -26,22 +26,14 @@ struct linux_binprm {
unsigned long p; /* current top of mem */
unsigned long argmin; /* rlimit marker for copy_strings() */
unsigned int
- /* Does bprm->file warrant clearing personality bits? */
- active_per_clear:1,
-
/* Should unsafe personality bits be cleared? */
per_clear:1,

/* Should an execfd be passed to userspace? */
have_execfd:1,

- /* It is safe to use the creds of a script (see binfmt_misc) */
- preserve_creds:1,
- /*
- * True if most recent call to security_bprm_set_creds
- * resulted in elevated privileges.
- */
- active_secureexec:1,
+ /* Use the creds of a script (see binfmt_misc) */
+ execfd_creds:1,
/*
* Set by bprm_creds_for_exec hook to indicate a
* privilege-gaining exec has happened. Used to set
diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 1e295ba12c0d..36b07c1eb0f1 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -50,7 +50,7 @@ LSM_HOOK(int, 0, settime, const struct timespec64 *ts,
const struct timezone *tz)
LSM_HOOK(int, 0, vm_enough_memory, struct mm_struct *mm, long pages)
LSM_HOOK(int, 0, bprm_creds_for_exec, struct linux_binprm *bprm)
-LSM_HOOK(int, 0, bprm_repopulate_creds, struct linux_binprm *bprm)
+LSM_HOOK(int, 0, bprm_creds_from_file, struct linux_binprm *bprm)
LSM_HOOK(int, 0, bprm_check_security, struct linux_binprm *bprm)
LSM_HOOK(void, LSM_RET_VOID, bprm_committing_creds, struct linux_binprm *bprm)
LSM_HOOK(void, LSM_RET_VOID, bprm_committed_creds, struct linux_binprm *bprm)
diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
index 62e60e55cb99..0aeaa3de69b2 100644
--- a/include/linux/lsm_hooks.h
+++ b/include/linux/lsm_hooks.h
@@ -46,18 +46,19 @@
* bits must be cleared from current->personality.
* @bprm contains the linux_binprm structure.
* Return 0 if the hook is successful and permission is granted.
- * @bprm_repopulate_creds:
- * Assuming that the relevant bits of @bprm->cred->security have been
- * previously set, examine @bprm->file and regenerate them. This is
- * so that the credentials derived from the interpreter the code is
- * actually going to run are used rather than credentials derived
- * from a script. This done because the interpreter binary needs to
- * reopen script, and may end up opening something completely different.
+ * @bprm_creds_from_file:
+ * If @bprm->file is setpcap, suid, sgid or otherwise marked to
+ * change the privilege level upon exec update @bprm->cred to
+ * handle the marking on the file. This is called after finding
+ * the native code binary that will be executed. This ensures that
+ * the credentials will not be derived from a script that the binary
+ * will need to reopen, which when reopend may end up being a completely
+ * different file.
* This hook may also optionally check permissions (e.g. for
* transitions between security domains).
- * The hook must set @bprm->active_secureexec to 1 if AT_SECURE should be set to
+ * The hook must set @bprm->secureexec to 1 if AT_SECURE should be set to
* request libc enable secure mode.
- * The hook must set @bprm->active_per_clear to 1 if the dangerous personality
+ * The hook must set @bprm->per_clear to 1 if the dangerous personality
* bits must be cleared from current->personality.
* @bprm contains the linux_binprm structure.
* Return 0 if the hook is successful and permission is granted.
diff --git a/include/linux/security.h b/include/linux/security.h
index 6dcec9375e8f..df8ad2fb7374 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -140,7 +140,7 @@ extern int cap_capset(struct cred *new, const struct cred *old,
const kernel_cap_t *effective,
const kernel_cap_t *inheritable,
const kernel_cap_t *permitted);
-extern int cap_bprm_repopulate_creds(struct linux_binprm *bprm);
+extern int cap_bprm_creds_from_file(struct linux_binprm *bprm);
extern int cap_inode_setxattr(struct dentry *dentry, const char *name,
const void *value, size_t size, int flags);
extern int cap_inode_removexattr(struct dentry *dentry, const char *name);
@@ -277,7 +277,7 @@ int security_syslog(int type);
int security_settime64(const struct timespec64 *ts, const struct timezone *tz);
int security_vm_enough_memory_mm(struct mm_struct *mm, long pages);
int security_bprm_creds_for_exec(struct linux_binprm *bprm);
-int security_bprm_repopulate_creds(struct linux_binprm *bprm);
+int security_bprm_creds_from_file(struct linux_binprm *bprm);
int security_bprm_check(struct linux_binprm *bprm);
void security_bprm_committing_creds(struct linux_binprm *bprm);
void security_bprm_committed_creds(struct linux_binprm *bprm);
@@ -575,9 +575,9 @@ static inline int security_bprm_creds_for_exec(struct linux_binprm *bprm)
return 0;
}

-static inline int security_bprm_repopulate_creds(struct linux_binprm *bprm)
+static inline int security_bprm_creds_from_file(struct linux_binprm *bprm)
{
- return cap_bprm_repopulate_creds(bprm);
+ return cap_bprm_creds_from_file(bprm);
}

static inline int security_bprm_check(struct linux_binprm *bprm)
diff --git a/security/commoncap.c b/security/commoncap.c
index 0b72d7bf23e1..2bd1f24f3796 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -797,22 +797,22 @@ static inline bool nonroot_raised_pE(struct cred *new, const struct cred *old,
}

/**
- * cap_bprm_repopulate_creds - Set up the proposed credentials for execve().
+ * cap_bprm_creds_from_file - Set up the proposed credentials for execve().
* @bprm: The execution parameters, including the proposed creds
*
* Set up the proposed credentials for a new execution context being
* constructed by execve(). The proposed creds in @bprm->cred is altered,
* which won't take effect immediately. Returns 0 if successful, -ve on error.
*/
-int cap_bprm_repopulate_creds(struct linux_binprm *bprm)
+int cap_bprm_creds_from_file(struct linux_binprm *bprm)
{
+ /* Process setpcap binaries and capabilities for uid 0 */
const struct cred *old = current_cred();
struct cred *new = bprm->cred;
bool effective = false, has_fcap = false, is_setid;
int ret;
kuid_t root_uid;

- new->cap_ambient = old->cap_ambient;
if (WARN_ON(!cap_ambient_invariant_ok(old)))
return -EPERM;

@@ -826,7 +826,7 @@ int cap_bprm_repopulate_creds(struct linux_binprm *bprm)

/* if we have fs caps, clear dangerous personality flags */
if (__cap_gained(permitted, new, old))
- bprm->active_per_clear = 1;
+ bprm->per_clear = 1;

/* Don't let someone trace a set[ug]id/setpcap binary with the revised
* credentials unless they have the appropriate permit.
@@ -889,7 +889,7 @@ int cap_bprm_repopulate_creds(struct linux_binprm *bprm)
(!__is_real(root_uid, new) &&
(effective ||
__cap_grew(permitted, ambient, new))))
- bprm->active_secureexec = 1;
+ bprm->secureexec = 1;

return 0;
}
@@ -1346,7 +1346,7 @@ static struct security_hook_list capability_hooks[] __lsm_ro_after_init = {
LSM_HOOK_INIT(ptrace_traceme, cap_ptrace_traceme),
LSM_HOOK_INIT(capget, cap_capget),
LSM_HOOK_INIT(capset, cap_capset),
- LSM_HOOK_INIT(bprm_repopulate_creds, cap_bprm_repopulate_creds),
+ LSM_HOOK_INIT(bprm_creds_from_file, cap_bprm_creds_from_file),
LSM_HOOK_INIT(inode_need_killpriv, cap_inode_need_killpriv),
LSM_HOOK_INIT(inode_killpriv, cap_inode_killpriv),
LSM_HOOK_INIT(inode_getsecurity, cap_inode_getsecurity),
diff --git a/security/security.c b/security/security.c
index b890b7e2a765..0688359bf8f4 100644
--- a/security/security.c
+++ b/security/security.c
@@ -828,9 +828,9 @@ int security_bprm_creds_for_exec(struct linux_binprm *bprm)
return call_int_hook(bprm_creds_for_exec, 0, bprm);
}

-int security_bprm_repopulate_creds(struct linux_binprm *bprm)
+int security_bprm_creds_from_file(struct linux_binprm *bprm)
{
- return call_int_hook(bprm_repopulate_creds, 0, bprm);
+ return call_int_hook(bprm_creds_from_file, 0, bprm);
}

int security_bprm_check(struct linux_binprm *bprm)
--
2.25.0
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